1. Field of the Invention
The present invention relates to apparatus for forming plastic sheet and, more particularly, to accurately thermoforming plastic sheet in registration with a mold.
2. Description of Related Art
Cold forming of plastic sheet material in conformance with a mold has the benefit of not requiring delays for cooling purposes associated with thermoforming sheet material at a temperature in excess of the transition (T.sub.g) temperature. However, cold forming requires significant pressures in the range of 50-300 bars depending upon the characteristics of the sheet material and the nature of the mold. Moreover, the resulting stretching of material, particularly for deep drawn plastic pieces, usually results in distortion of any indicia disposed upon the sheet material. Accordingly, the indicia must be applied subsequent to the cold forming process or through use of an added sheet material containing the indicia.
Thermoforming sheet material by conventional heating of the sheet to at least the softening temperature produces sag of the material prior to contact with a mold. Such sag may distort any indicia disposed upon the sheet. Moreover, the heating is usually not uniform across the sheet and will produce cold spots and hot spots. The uneven heating will result in uneven stretching of the material over a mold.